There hasn’t been strong evidence to support the idea that vitamins can combat cancer—until now.

In the first rigorous, long-term study of multivitamins and their effect on cancer, older men who took daily vitamins lowered their risk of cancer by 8% compared to men who skipped the supplements over an average of 11 years of follow up.

Participants included 14,641 male U.S. physicians ages 50 and over who were enrolled in the Physicians’ Health Study for 11 years. The men were randomly assigned to take a multivitamin —Centrum Silver — or a placebo, and neither they nor the scientists were aware of their status. Overall, they were healthy; two-thirds exercised on a regular basis and only 4% were current smokers. The doctors’ mean age was 64. Nine percent (1,312) reported a history of cancer (excluding nonmelanoma skin cancer).

“It appears that there may be a modest benefit in preventing cancer in men over the age of 50,” said Dr. J. Michael Gaziano, lead author of the study published in the Journal of the American Medical Association (JAMA), and chief of the Division of Aging at Brigham and Women’s Hospital, in a teleconference call. The findings were presented Wednesday at an American Association for Cancer Research conference in Anaheim, Calif.

Previous studies haven’t been so definitive, which is why the 2010 Dietary Guidelines for Americans and the National Institutes of Health both decided that the evidence wasn’t strong enough to advise healthy people to take vitamins daily. One placebo-controlled study published in JAMA revealed that taking large doses of vitamin D supplements did not prevent colds or reduce symptoms in healthy adults any more than a placebo. Another analysis of clinical trials found that omega-3 supplements do not reduce users’ risks of heart attack, stroke, or death from heart disease. And older women who took daily multivitamins were 6% more likely to die over a 19-year-period compared to those who did not take the pills. Higher odds of death were associated with vitamin B, folic acid, iron, magnesium, zinc, and copper supplements; calcium, however, was associated with a 10% lower risk of death in the women.

But because the participants in Physicians’ Health Study were randomly assigned to take vitamins or placebo, and because they were followed for a relatively long period of time, the findings may finally provide some clarity on the role that vitamins can play in suppressing cancer.

That doesn’t mean that vitamins are the antidote to cancer, or that they can offset cancer-causing behaviors like smoking or an unhealthy diet. It also doesn’t mean that vitamins are the only way to fight cancer. Skipping the vitamins and getting the same nutrients from a well-balanced diet can also be an effective way to keep tumors at bay. “A varied diet is associated with reduced risk of cancer, so a multivitamin may mimic the vitamins and minerals we get from that varied diet,” Gaziano said.